(1.) Field of the Invention
This invention relates to a resin composition used for a vibration-damping material such as a composite-type vibration-damping steel sheet comprising a resin layer provided between two steel sheets, a composite-type vibration-damping material using the resin composition, a composite-type vibration-damping material having spot weldability, and process for production thereof. Particularly, the invention relates to a composite-type vibration-damping material having an excellent vibration-damping effect at or around normal temperature, as well as high adhesion performance, a resin composition which enables the production of a composite-type vibration-damping material having spot weldability in addition to the above-mentioned properties, a composite-type vibration-damping material using the resin composition, and process for production thereof. The composite-type vibration-damping material thus obtained can be used as a noise and vibration-reducing material. The composite-type vibration damping material is applicable, as noise and vibration-reducing material, not only to building material for stairways, doors, floor materials, etc., but to automotive oil pan, dashboard, floor panel, roof panel and other trim parts, to which the conventional composite-type vibration-damping materials could not be easily applied. Also, the composite-type vibration-damping material according to this invention can be used for motor and compressor covers, etc. Thus, the composite-type vibration-damping material according to the invention can be used widely in the automobile industry, civil engineering and construction industries as well as electrical machinery industry.
(2.) Description of the Prior Art
A composite-type vibration-damping steel sheet is a noise and vibration-reducing material which comprises a layer of a viscoelastic resin (hereinafter referred to as "the intermediate resin") provided between two steel sheets so that vibrations exerted on the steel sheets are converted into thermal energy by the intermediate resin layer. The use of the composite-type vibration-damping steel sheets for automotive oil pans, stairways, doors, floor material or other building materials, motor or compressor covers, etc., has been made or investigated, in view of the recent needs for noise control.
Generally, the vibration-damping performance of such a composite-type vibration-damping material depends on the performance of the intermediate resin layer. It is known that the vibration-damping performance, represented by loss factor (.eta.), shows a peak at a specific temperature, and the vibration-damping material produces its greatest effect when used in the vicinity of the peak characteristic temperature.
The vibration-damping performance is not only temperature-dependent but is dependent on the frequency of the source of noise or vibration. Generally, the loss factor is higher as the frequency is higher.
For use as the intermediate resin in the vibration-damping steel sheet, a variety of materials have been studied, for instance, polyurethane (Japanese Patent Application Laid-Open (KOKAI) No. 47-19277 (1972)), vinylurethane resin (Japanese Patent Laid-Open (KOKAI) No. 50-39737 (1975), polyester resin (Japanese Patent Application Laid-Open (KOKAI) No. 50-143880 (1975)), polyamide resin (Japanese Patent Application Laid-Open (KOKAI) No. 51-79146 (1976)), polyisobutylene (Japanese Patent Application Laid-Open (KOKAI) No. 54-43251 (1979)), ethylene/.alpha.-olefin resin (Japanese Patent Application Laid-Open (KOKAI) No. 55-84655 (1980)), EVA (Japanese Patent Application Laid-Open (KOKAI) No. 57-34949 (1982)), crosslinked polyolefin (Japanese Patent Application Laid-Open (KOKAI) No. 59-152847 (1984)), polyvinyl acetal (Japanese Patent Application Laid-Open (KOKAI) No. 60-88149 (1985)), etc. Besides, it has been known that asphalt, synthetic rubbers, acrylic adhesives, epoxy resins and the like also have a vibration-damping property. Of these materials, the resins elastic at room temperature, such as acrylic adhesives, isobutylene rubber, EVA, etc., have comparatively high vibration-damping properties at temperatures near normal temperature. These elastic resins however, are poor in adhesive strength due to their low cohesive force at normal temperature, so that the vibration-damping steel sheets using the resins are incapable of enduring press forming and processing. Further, vibration-damping steel sheet materials subjected to press working may be subsequently subjected to a baking finish step in which they are heated to about 200.degree. C. Thus, it is necessary that the intermediate resin should not flow out in the vicinity of the temperature and, also, the lowering in the adhesive strength should be little. Therefore, the above-mentioned vibration-damping steel sheets, naturally lacking such a heat resistance, have been applied only to building material uses where they have been used in substantially flat forms.
On the other hand, polyolefin resins modified by copolymerization, blending or the like, for instance, ethylene/.alpha.-olefin resins, are comparatively better in vibration-damping property at higher temperature of 50.degree. to 100.degree. C. and stronger in cohesive force at normal temperature, as compared with the above-mentioned elastic resins, and are able to endure press forming and processing. Therefore, the vibration-damping steel sheets using the modified polyolefin resin have been said to be suitable for application to high-temperature uses, such as automotive oil pans.
However, the above-mentioned resins (and the vibration-damping steel sheets using them) are still unsatisfactory in the vibration-damping performance and adhesion performance. Moreover, there has not yet been, found out any resin that is suitable for a vibration-damping steel sheet required to have a high vibration-damping performance at or around normal temperature, furthermore to have a high vibration-damping performance even in a low frequency region, as in application to automotive trim parts, and to show strong adhesion during and after the press forming and processing thereof as well as good heat resistance of adhesion at high temperatures.
Use of polyester resin as the resin for a vibration-damping steel sheet has hitherto been studied. Examples of the studies include the following:
(1) a so-called urethane foam obtained by preparing a urethane prepolymer from a polyester resin of a molecular weight of up to 1800 and an isocyanate compound, and using an amine compound or the like as a curing agent (Japanese Patent Application Laid-Open (KOKAI) No. 47- 19277 (1972));
(2) a polyester resin having a glass transition temperature of 0.degree. to 60.degree. C., prepared from a glycol and a phthalic acid (Japanese Patent Application Laid-Open (KOKAI) No. 50-143880 (1975);
(3) a polyester resin having a specified elongation and a specified peak temperature of loss factor (Japanese Patent Application Laid-Open (KOKAI) No. 61-277435 (1986));
(4) a mixture of a polyester resin and a polyolefin resin (Japanese Patent Application Laid-Open (KOKAI) No. 61-89842 (1986));
(5) a composition obtained by blending two amorphous polyester resins differing in composition (Japanese Patent Application Laid-Open (KOKAI) No. 62-295949 (1987));
(6) a composition comprising a polyester diol (A) which has a molecular weight of 600 to 6000 and in which at least 60 mol % of the dicarboxylic acid component comprises an aromatic dicarboxylic acid component and glycol, an aliphatic polyester diol (B) having a molecular weight of 600 to 6000, and a diisocyanate compound (Japanese Patent Application Laid-Open (KOKAI) No. 63-48321 (1988));
(7) a composition comprising a polyester diol (A) which has a molecular weight of 600 to 6000 and in which at least 60 mol % of the dicarboxylic acid component comprises an aliphatic dicarboxylic acid of 4 to 20 carbon atoms, a chain extender (B) having a molecular weight of up to 400, and a diisocyanate compound (Japanese Patent Application Laid-Open (KOKAI) No. 63-202613 (1988)); and
(8) a composition comprising an amorphous polyester having a maximum of loss tangent (tan.delta.) of at least 0.5 in the temperature range of from -40.degree. to 120.degree. C., an acid anhydride, and an epoxy compound (Japanese Patent Application Laid-Open (KOKAI) No. 63-75056 (1988)).
However, the urethane foam of (1) above is poor vibration-damping properties, and is unsatisfactory in adhesion performance.
The resin of (2) is insufficient in adhesion performance at normal temperature, and lacks heat resistance and durability, for instance at about 100.degree. C. Thus, neither of the resins of (1) and (2) has satisfactory performance as a resin for a vibration-damping material.
The resin materials of (3), (4) and (5) above show high vibration-damping properties in specified temperature ranges in which they are viscoelastic, but their cohesive forces are insufficient. Therefore, the resin materials are poor in adhesive force even at temperatures at which they show practical vibration-damping properties, and, at a higher temperature, they naturally cannot have satisfactory strength. Thus, none of the resin materials of (3), (4) and (5) have satisfactory performance as a resin for a vibration-damping material.
The resin composition of (6) overcomes the above-mentioned drawback of insufficient cohesive force, and shows a comparatively high adhesive strength and loss factor. However, the temperature corresponding to the maximum vibration-damping performance of the resin composition is so high that the vibration-damping property at around normal temperature is not good, and the resin composition does not have sufficient performance as a resin for a vibration-damping material which produces the desired effect in the vicinity of normal temperature, especially in the vicinity of normal temperature as well as in a low-frequency region. Furthermore, the resin composition shows remarkable drop in adhesion strength after painting and baking at about 200.degree. C.
The resin composition of (7) has a high vibration-damping property at or around normal temperature. Due to the low molecular weights of the components (A) and (B) and to the aliphatic materials constituting the resin composition, the resin composition is insufficient in adhesion performance at normal temperature, and lacks resistance to hydrolysis. Further, the resin composition shows a marked reduction in adhesive force when exposed to a high temperature, for instance about 200.degree. C., and is not capable of satisfactory use as a resin for a vibration-damping material.
The resin composition of (8) is shown to have high adhesive strength and vibration-damping property at around normal temperature. However, there is a description that when the amorphous polyester resin is used solely or with an isocyanate crosslinking agent, it is difficult to achieve a high adhesive strength under shear, which has an important effect on press-workability. Therefore, resin compositions comprising the polyester and the isocyanate crosslinking agent have not yet reached a satisfactory level of usability as a resin for a vibration-damping material.
In short, in composite-type vibration-damping materials produced by use of the conventional intermediate resins mentioned above, the resins do not function satisfactorily as a resin for a vibration-damping material which exhibits high vibration-damping performance particularly at around normal temperature and which has high adhesion and durability, furthermore which exhibits high vibration-damping performance against vibrations in a low-frequency region.